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CERN, Feb.17, 2009Practical operation of Micromegas3 Micromegas: How does it work? Y. Giomataris, Ph. Rebourgeard, JP Robert and G. Charpak, NIM A 376 (1996) 29 S1 S2 Micromesh Gaseous Chamber: a micromesh supported by  m insulating pillars, and held at V anode – 400 V one stage Multiplication (up to 10 5 or more) takes place between the anode and the mesh and the charge is collected on the anode (one stage) transparency Funnel field lines: electron transparency very close to 1 for thin meshes fast Small gap: fast collection of ions S2/S1 = E drift /E amplif ~ 200/60000= 1/300

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CERN, Feb.17, 2009Practical operation of Micromegas 4

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CERN, Feb.17, 2009Practical operation of Micromegas5 Small size => Fast signals => Short recovery time => High rate capabilities micromesh signal strip signals A GARFIELD simulation of a Micromegas avalanche (Lanzhou university) Electron and ion signals seen by a fast (current) amplifier In a TPC, the signals are usually integrated and shaped

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CERN, Feb.17, 2009Practical operation of Micromegas7 200  m MESHES Electroformed Chemically etched Wowen PILLARS Deposited by vaporization Laser etching, Plasma etching… Many different technologies have been developped for making meshes (Back-buymers, CERN, 3M-Purdue, Gantois, Twente…) Exist in many metals: nickel, copper, stainless steel, Al,… also gold, titanium, nanocristalline copper are possible. Can be on the mesh (chemical etching) or on the anode (PCB technique with a photoimageable coverlay). Diameter 40 to 400 microns. Also fishing lines were used (Saclay, Lanzhou)

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CERN, Feb.17, 2009Practical operation of Micromegas8 The Bulk technology Fruit of a CERN-Saclay collaboration (2004) Mesh fixed by the pillars themselves : No frame needed : fully efficient surface Very robust : closed for > 20 µ dust Possibility to fragment the mesh (e.g. in bands) … and to repair it Used by the T2K TPC under construction

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CERN, Feb.17, 2009Practical operation of Micromegas10 ‘ Choose your material For first tests of a detector, a power supply with current limitation is preferred. Set the current limitation at 500 nA for instance. The CAEN N471A is ideal for testing, though not very precise. They have 2 chanels, you can use one for the mesh and one for the drift cathode. Check your gasbox for gas-tightness : must bubble down to 1 l/h. Before connecting the electronics, ‘cook’ your detector (see next slide). Preamp: use a protected fast preamp (for instance ORTEC 142 series) and an amplifier-shaper (0.5 or 1 microsecond peaking time), for instance ORTEC 472 or 672. Hunt noise (microphonic noise, radiated noise, noise from the grounds)

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CERN, Feb.17, 2009Practical operation of Micromegas11 ‘ Burning’ or ‘cooking’ your detector To make the detector stable for further operation, it must be ‘cooked’ : raise the voltage slowly to V (50 micron gap) or V (128 micron gap), step by step, to the level where it starts sparking. This has to be done in air It consists of burning small dusts (mostly fibres). A relatively high (ionic) current ( nA) can remain. It will decrease after circulation of the gas and go down to 0(1nA). A detector which stands its voltage in air will always work in gas.